There are more faces involved than the ones mentioned in the link you pointed to. The answer is to use C-u C-x = to find out what faces are involved. Those are the ones you need to customize.
Use M-x customize-face to customize the faces you are interested in. If you don't know which faces they are, put the cursor on that highlighted text and use C-u C-x =. Look near the bottom of *Help* for the face name.
From @djangoliv's answer it seems that these are the faces you need to customize: diff-refine-added, diff-refine-removed, diff-refine-changed.
So just M-x customize-face diff-refine-added and define the face as you like.
Do the same for diff-refine-removed and diff-refine-changed.
Easy, persistent, and the way Emacs wants to help you customize faces. See the Emacs manual, node Specific Customization.